The present invention relates to a welding apparatus and, more particularly, to a projection welder constructed to facilitate an easy positioning of a weld nut to be welded to a work.
Many kinds of projection welders have been known which are intended to weld nuts to inner surfaces of works having cross-sections of U-shape, H-shape, E-shape, etc. to form projections on the work inner surfaces and which are equipped with nut feeders operative to automatically feed weld nuts to lower electrodes of the welders. Examples of such projection welders are disclosed in Japanese Patent Pre-Examination Publication Nos. 32,846/77 and 116,750/77 and Japanese Utility Model Publication No. 14,943/81.
More particularly, the Publication No. 32,846/77 discloses a projection welder having a lower electrode tip formed therein with a longitudinal groove along which weld nuts are fed one after another by a push rod to a predetermined position. The publication No. 116,750/77 discloses a projection welder of the design described above and which is constructed to direct a jet of air to a nut to be welded to a work. The third publication 14,943/81 discloses a projection welder provided with a nut feeder disposed obliquely relative to a work and having a feeding member adapted to magnetically hold a nut on the forward end thereof and feed the nut to a welding portion of the work.
In all of the prior art projection welders, weld nuts must be fed to lower electrodes with projections of the nuts directed toward mating works so that the projections of the nuts are welded to the inner surfaces of the works. For this purpose, a lower electrode has been known which is provided with a so-called "base type" nut feeder operative to automatically feed weld nuts to the electrode.
The method of projection welding is classified into two kinds as follows:
(a) A projection welding method in which a hole is preliminarily formed in a work at a portion to be welded to a nut so that the hole is utilized to center the nut to the welding portion of the work; and
(b) A "no-hole" projection welding method in which no hole is preliminarily formed in a work but the work is first welded to a nut and thereafter a punch disposed in an upper electrode cooperates with the welded nut to punch a hole in the welded work. In the "no-hole" method, the welded nut is utilized as a die which cooperates with the punch disposed in the upper electrode. Thus, nuts must be procisely positioned in alignment with the upper electrode in successive projection welding operations because, if the nuts are out of alignment with the upper electrode and thus with the punch disposed therein, cutting edges of the punch and nuts are unable to accurately punch holes in works with resultant production of inferior goods and occurrence of damage in the electrodes.
In the projection welders for welding nuts to the inner surfaces of works having such a complicated cross-section as referred to above, operators can neither confirm the presence or absence of a nut in position under a work nor watch the position of the nut relative to the work. It is, therefore, required to reliably feed weld nuts successively to a lower electrode of a projection welder and to precisely position the nuts to welding portions of successive works in order that projection welding may be carried out automatically or without control by any operator.
The prior art, however, has following problems:
(a) In the projection welder having the base type lower electrode, weld nuts are successively fed through a feeding groove by a push rod reciprocally movable in the groove. The feeding groove has a width greater than the outer dimension of individual weld nuts so that the nuts can be fed through the groove. In addition, the lower electrode has a nut positioning portion having a dimension greater than the outer dimension of individual weld nuts. As such, the nut-feeding groove and the nut-positioning portion of the lower electrode are determined to facilitate easy feeding and positioning of weld nuts, irrespective of dimensional fluctuation of nuts to be fed. However, the nut-feeding groove and the nut-positioning portion of the lower electrode are obliged to be deformed and widened not only due to wear by sliding engagement with nuts to be fed but also due to heat and pressure generated during repeated projection welding operations.
As such, the positioning of weld nuts in the base type lower electrode depends upon the outer configurations and dimensions of weld nuts rather than on the centers of threaded holes in the nuts which are to be centered relative to the upper electrode. Thus, in the case where the threaded holes in the weld nuts are not coaxial with the outer configurations of the nuts and in the event that the nut-feeding groove and the nut-positioning portion of the lower electrode have widthwise dimensions different from the outer dimensions of weld nuts to be centered to the upper electrode, the nut-feeding groove and/or the nut-positioning portion will fail to correctly center the nuts relative to the upper electrode. This adversely affects the punching in the no-hole projection welding method.
(b) The lower electrodes of the base type are classified into two styles in one of which the lower electrode and the nut-feeding groove member are integral, as disclosed in the above-referenced Japanese Publication Nos. 32,846/77 and 116,750/77 and in the other of which the lower electrode tip, the nut-feeding section and the nut-positioning section are detacheably or removably assembled.
In the first style, all the elements are formed of an electrically insulating material and, if the lower electrode section and the nut-feeding section are worn or deformed, it is difficult to repair only the worn or deformed section. In such an event, therefore, the lower electrode must be replaced as a whole by a new one. It will be appreciated that any electrically insulating materials are relatively soft and tend to be quickly worn. Thus, the lower electrode must be frequently replaced with another, which decreases the economy of welding operation.
In the second style of the lower electrode, worn or deformed parts can easily be replaced by new ones because the lower electrode tip and the nut-feeding and positioning sections are detachably assembled. This, however, greatly complicates the lower electrode structure and increases the length of time required for maintenance. In addition, the nut-positioning section must be made of a material sufficiently hard enough to prevent deformation which would otherwise be caused by the heat produced each time when a work and a nut are welded together. Thus, the second style of lower electrode is expensive.
In addition, the lower electrode is usually associated with a loader for works and various kinds of detection mechanisms for automatic welding operations. Thus, the increase in the size of the lower electrode and the complication of the structure thereof result in a very complicated and bulky welding system as a whole.
(c) Each time when a work is welded to a nut by projection welding, a welding spatter is produced much or less and tends to be adhered to threaded surface of the nut welded to the work, which tends to result in the production of a inferior goods.
In a projection welder operated by the no hole method and provided with a lower electrode of the base type, the weld spatter is directed mainly to the outer surface of a nut welded to a work. However, a part of the spatter is directed not only into a threaded hole in the nut and adhered to the threaded inner peripheral surface of the hole to deteriorate the quality of the threaded hole, but also into an annular groove of U-shaped cross-section which is designed to cooperate with an upper punch to punch a hole in the work welded to the nut with a result that the groove is filled partly or fully with the welding spatter and becomes inoperative to cooperate with the punch.
The problems discussed above are serious in automating welding steps and especially in attaining a highly automated and highly efficient no-hole projection welding method. Thus, there has been a demand for a projection welder in which nuts are fed automatically, precisely and efficiently.